The present invention relates to an electrophotographic photosensitive member with improvements in chargeability, photosensitivity, and environmental durability.
Organic and inorganic materials have conventionally been used to form photoconductive layers of electrophotographic photosensitive members. Among the inorganic materials are CdS, ZnO, selenium, Se-Te system, and amorphous silicon. The organic materials include poly-N-vinyl carbazole (PVCz) and trinitrofluorenone (TNF). In the photosensitive members using these photoconductive materials, however, there are various problems related to their photoconductive characteristics and workmanship. Therefore, the characteristics of the photosensitive system have been sacrificed, in some measure, in using these materials properly, according to the applications.
For example, selenium and CdS are harmful to health, and must be prepared with special care for safety's sake. Accordingly, they require complicated manufacturing apparatuses and thereby entail high production costs. In particular, selenium must be recovered and this necessitates additional cost. Moreover, selenium and Se-Te system, whose crystallization temperature is as low as 65.degree. C., will be confronted with problems related to their photoconductive characteristics, such as residual potential, during repeated copying operations. Therefore, they are short-lived and not very practical.
Further, ZnO is not reliable in use because it is liable to oxidize or reduce, and is highly susceptible to the environmental atmosphere.
It is suspected, furthermore, that organic photoconductive materials such as PVCz and TNF are carcinogens. Besides being harmful to health, they are low in thermal stability and in wear resistance, and are therefore short-lived.
Meanwhile, amorphous silicon (hereinafter abbreviated to a-Si) is a photoconductive material which has recently become the object of public attention. It is frequently applied to solar cells, thin-film transistors, and image sensors. As a part of such applications, a-Si has been tried as a material for electrophotographic photosensitive members. Since a-Si produces no pollutant, photosensitive members formed of a-Si need not be recovered. Also, they have higher spectral sensitivity in the visible radiation area than those made of other materials, and are high in surface hardness, wear resistance, and shock resistance.
Amorphous silicon is being studied as a material in photosensitive members for electrophotography, based on the Carlson process. In this connection, the members must have high resistance and photosensitivity. It is difficult, however, for a single-layer photosensitive member to provide both these characteristics. To meet these requirements, therefore, laminate-type photosensitive members have been developed which are constructed such that a barrier layer is sandwiched between a photoconductive layer and a conductive substrate, and a surface charge retentive layer is formed on the photoconductive layer.
Usually, a-Si is formed by the glow discharge decomposition process, using silane gas. In this process, hydrogen is trapped in an a-Si film, so that the electrical and optical characteristics of the film vary considerably, depending on the hydrogen content. If the amount of hydrogen incorporated in the a-Si film increases, the optical band gap becomes greater, increasing the resistance of the film, so that the film has reduced sensitivity to long wavelength light. It is therefore difficult to use it suitably, for example, in a laser beam printer mounted with a semiconductor laser. If the hydrogen content of the a-Si film is high, (SiH.sub.2).sub.n and other bonds may sometimes occupy the greater part of the film, depending on the filming conditions. Thereupon, voids spread, thus increasing silicon dangling bonds, lowering the photoconductive characteristics. Thus, the film cannot be used for an electrophotographic photosensitive member. If the amount of hydrogen incorporated in the a-Si film is reduced, on the other hand, the optical band gap is narrowed, reducing the resistance of the film, although the film now has increased sensitivity to longer-wave light. If the hydrogen content is low, however, less hydrogen links with the silicon dangling bonds, thereby reducing them. Accordingly, the mobility of resulting carriers is reduced, and the film is lowered in life performance and photoconductive characteristics. Thus, the film becomes unfit for use in the photosensitive member.
In a conventional method of increasing the sensitivity to longer-wave radiation, silane gas is mixed with germane (GeH.sub.4), and is subjected to glow discharge decomposition, thereby forming a film with a narrow optical band gap. In general, silane gas and GeH.sub.4 are different in the optimum substrate temperature, so that the resultant film is subject to many structural defects and cannot provide satisfactory photoconductive characteristics. Moreover, waste gas from GeH.sub.4 becomes poisonous when it is oxidized, requiring a complicated processing system therefor. In consequence, this technique is not practical.